Remove common whitespace prefix from src block lines.

src/intermediate/src_block.rs

@@ -14,12 +14,60 @@ intermediate!(
     original,
     _registry,
     {
-        let lines = original
-            .get_value()
+        let source_code = original.get_value();
+        let prefix_content_pairs: Vec<_> = source_code
             .split_inclusive('\n')
-            .map(|s| s.to_owned())
+            .map(|line| {
+                line.find(|c: char| !c.is_ascii_whitespace() && c != '\n')
+                    .map(|content_start| line.split_at(content_start))
+                    .map(|(leading_whitespace, content)| {
+                        (
+                            leading_whitespace
+                                .chars()
+                                .map(ascii_whitespace_value)
+                                .sum::<usize>(),
+                            content,
+                        )
+                    })
+                    .unwrap_or((0, line))
+            })
+            .collect();
+        let common_whitespace_prefix = prefix_content_pairs
+            .iter()
+            .filter_map(|(leading_whitespace, _content)| {
+                if *leading_whitespace > 0 {
+                    Some(*leading_whitespace)
+                } else {
+                    None
+                }
+            })
+            .min()
+            .unwrap_or(0);
+        let lines: Vec<_> = prefix_content_pairs
+            .into_iter()
+            .map(|(leading_whitespace, content)| {
+                let mut line = String::with_capacity(common_whitespace_prefix + content.len());
+                if leading_whitespace > common_whitespace_prefix {
+                    // Empty lines can have a leading_whitespace less than common_whitespace_prefix.
+                    for _ in 0..(leading_whitespace - common_whitespace_prefix) {
+                        line.push(' ');
+                    }
+                }
+                line.push_str(content);
+                line
+            })
             .collect();
         let language = original.language.map(str::to_owned);
         Ok(ISrcBlock { lines, language })
     }
 );
+
+fn ascii_whitespace_value(c: char) -> usize {
+    match c {
+        ' ' => 1,
+        '\t' => 8,
+        '\r' | '\n' => 0,
+        c if c as u32 == 12 => 0, // form feed
+        _ => unreachable!("Only ascii whitespace can reach this code."),
+    }
+}